[Users] Patch: allow custom LORENE_LIBS in optionlist

Hello,

at the moment the compilation of EinsteinToolkit with LORENE fails if LORENE is configured to use FFTW as LORENE_LIBS does not contain fftw3.

The attached patch solves the problem by allowing the user to overwrite LORENE_LIBS in the optionlist file. If LORENE_LIBS is not set then the old values are used, so that this modification should not break existing configurations where fftw3 could be not available.

Could you please consider applying the attached patch to the LORENE thorn in the development branch?

David